Research on reducing inter-cell interference is actively conducted in a wireless communication system to provide users with a better Quality of Service (QoS). Accordingly, a scheme for reducing inter-cell interference through cooperation between Base Stations (BS) is being researched. The scheme for reducing inter-cell interference through cooperation between the BSs may result in performance deterioration when a time delay occurs according to an amount of information exchanged between the BSs. Therefore, it is important to exchange a small amount of information when the BSs cooperate with each other.
Precoding Matrix Index (PMI) restriction, or PMI recommendation, is one exemplary scheme for cooperation between BSs. The PMI restriction is a scheme in which a Mobile Station (MS) estimates a precoding matrix (or vector) of a neighbor BS expected to have a great interference to the MS itself in a wireless communication system, so that the estimated precoding matrix is restricted not to be used when the neighbor BS performs beamforming on downlink information.
FIG. 1 illustrates a structure of a system using a conventional PMI restriction scheme.
Referring to FIG. 1, for the PMI restriction scheme, an MSA 110 estimates a downlink channel HAA 120 with respect to a serving BSA 100 and selects a precoding matrix ν1A 1A having a greatest gain for the MSA 110 from precoding matrixes ν1A used by the serving BSA 100. Further, the MSA 110 estimates a downlink channel HAB 122 with respect to a neighbor BSB 102 and selects a precoding matrix ν2B that is expected to have a greatest interference to the MSA 110 from precoding matrixes νiB used by the neighbor BSB 102. Thereafter, the MSA 110 reports information regarding the selected precoding matrixes to the serving BSA 100. In this case, as shown in FIG. 2A, the MSA 110 selects a precoding matrix ν1A of an effective channel HAAν1A 201 having a greatest channel gain from effective channels HAAνiA as an allowed precoding matrix, and selects a precoding matrix ν2B of an effective channel HABν2B 203, which has a greatest influence on the HAAν1A 201 having the greatest channel gain, from expected interference effective channels HABνiB as a restricted precoding matrix.
Upon receiving information on the allowed precoding matrix ν1A and the restricted precoding matrix ν2B 130 from the MSA 110, the BSA 100 transmits the information regarding the restricted precoding matrix ν2B 130 to the neighbor BSB 102. Additionally, the neighbor BSB 102 transmits the received information regarding the restricted precoding matrix ν2B 130 to an MSB 112 included in the coverage of the neighbor BSB 102. In this case, the allowed precoding matrix ν1A is used when the BSA 100 transmits downlink data to the MSA 110.
The MSB 112 estimates a downlink channel HBB 126 with respect to its serving BSB 102, and selects a precoding matrix having a greatest gain for the MSB 112 from the precoding matrixes νiB used by the serving BSB 102 except for the restrictive precoding matrix ν2B. Thereafter, the MSB 112 transmits the selected precoding matrix to the serving BSB 102. In this case, as shown in FIG. 2B, the MSB 112 selects a precoding matrix ν3A of an effective channel HBBν3B 223 having a greatest gain from the effective channels HBBνiB except for an effective channel HBBν2B 221 corresponding to the restrictive precoding matrix.
If the MSB 112 selects the precoding matrix having the greatest gain from the remaining precoding matrixes except for the restrictive precoding matrix as described above, interference to the MSA 110 can be reduced. However, in this case, a problem exists wherein the MSB 112 experiences interference by data transmitted to the MSA 110. For example, as shown in FIG. 2B, in a state where only the effective channel HBBν2B 221 corresponding to the restrictive precoding matrix is excluded, the effective channel HBBν3B 223 having a greatest gain for the MSB 112 experiences a great interference by the interference effective channel HBAν1A 225 established by the precoding matrix ν1A selected by the MSA 110.